1977 Chevrolet Corvette 5-Speed

A change for the better.

If you follow drag racing, the name Doug Nash needs little introduction. To many quarter-mile racers, Doug Nash and high-performance transmissions are synonymous. Nash was a drag racer from 1963 until 1967 when he quit to open a small shop doing prototype engine work. Modifying engines evolved into modifying transmissions, an occupation Nash has pursued since 1971.

After years of modifying regular production components into pieces that would withstand the rigors of 8000-rpm dump-the-clutch drag racing starts and foot-to-the-floor speed shifts, Nash got to thinking how much easier it would be to design a competition transmission from the ground up. The result is what Nash calls the 4 + 1 Quick Change 5-speed manual gearbox. It's designed strictly for competition and features a lightweight magnesium alloy case that splits like a clamshell to facilitate gear changes and servicing, straight-cut spur gears offering higher strength and lower friction than conventional helical gears and a choice of 17 different ratios. Although the 4 + 1 Quick Change is primarily designed for straight-line competition, word of its advantages has spread to other forms of racing. Herb Adams, for example, is using a Nash 5-speed in his Silverbird Trans Am racer.

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Success on the race track has now led Nash to develop a street 5-speed transmission. The street box features an aluminum split-case design identical to the racing version and an overall weight of 95 lb, only 3 lb heavier than a standard Borg-Warner Super T-10 4-speed. Wider gears, larger gear teeth and high quality materials give the street 4 + 1 a considerable torque capacity advantage compared to an original-equipment 4-speed. In fact, Nash is so confident of the high quality of materials and workmanship that go into his 5-speed that he offers a 1-year warranty covering everything but the synchronizer mechanisms.

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Installing the Nash 5-speed is fairly straightforward because the case is the same width and the same height from the mainshaft centerline to the top of the case as are all popular American full-size 4-speed transmissions. Currently Nash builds all his street 5-speeds to the same overall length as the General Motors 4-speeds, but if the demand is there, he'll supply tail housings that make the 4 + 1 a bolt-in operation with and products also. In addition, the front of the gear case is drilled to mate with all standard factory bell housings and a variety of input and output splines are available so the 4 + 1 will bolt up to most standard clutches, driveshafts and front U-joints. Most GM 4-speeds have a transmission tailshaft mount further forward than the Nash 4 + 1 and require either a fabricated bracket or modifications to the original. However, Chrysler and Ford installations can use the stock mounting bracket.

Nash has a 2-fold reason for offering a street 5-speed. The first four ratios are numerically higher than those in available 4-speed gearboxes while 5th gear is 1.00:1, same as in conventional 4-speeds. The result is improved acceleration in the first four gears with no increase in engine speed for highway cruising and thus no reduction in fuel economy. This setup should prove popular with the stoplight Grand Prix set.

However, the typical R&T reader will be more interested in reason two. By the proper choice of a numerically lower final drive ratio combined with those numerically higher first four gears, it's possible to achieve the same overall torque multiplication as a conventional 4-speed coupled to a numerically higher rear end ratio. But the big difference is that when you shift into 5th, you get the fuel economy and engine speed and noise reduction advantages of the lower numerical axle ratio. In essence, an overdrive transmission.

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The Nash supplied to R&T for testing will serve as a numerical example. The overall gearing of the standard Corvette wide-ratio 4-speed combined with a 3.70:1 final drive is similar to the first four gears of the Nash 5-speed mated to a 3.08:1 rear end. Gene McCrickard, Nash's National Sales Director, drove the Corvette equipped with the high performance L82 350-cu-in V-8 and 5-speed out to California from Nash's home base in Tennessee, averaging 16.5 mpg for the 3000-mile trip. After R&T's performance, noise and subjective evaluations, McCrickard took the car to Ak Miller's garage in Pico Rivera where the 5-speed and 3.08:1 rear end were swapped for the wide-ratio Corvette gearbox and 3.70:1 ratio. Then all tests were repeated and the results compared. As expected from the similarity of the overall ratios in gears 1-4, there was no difference in quarter-mile performance. However, the top speed at 5500 rpm jumped from 116 mph with the 4-speed to 139. Also, the engine revs at 60 mph dropped from 2890 rpm with the 4-speed to 2370 with the 4 + 1. At speeds of 30, 50, 70 and 90 mph, our noise meter would measure no difference in the sound level between the 4- and 5-speeds, but subjectively that 520 rpm drop in engine speed at 60 mph made freeway driving much more pleasant.

On the trip back to Tennessee, McCrickard averaged 15.0 mpg with the 4-speed and 3.70:1 final drive so the 5-speed and 3.08:1 rear gear were worth about 1.5 mpg. In another test, a 1200-mile round trip from Tennessee to New York in a Pontiac Trans Am, McCrickard found a 2.5-mpg advantage for a Nash 5-speed and a 3.08:1 axle versus the stock 4-speed and a 3.40:1 final drive.

Our feelings about the 5-speed are a mixture of mostly pluses and a few minuses. No one cared for the howling noises the transmission made in the intermediate gears. "It sounds like a Muncie M22 Rock Crusher 4-speed," one staffer said. And he was more right than he realized at the time because gears 1-4 are cut at a 20-degree helix angle, the same as the Rock Crusher, instead of a more normal 25-degree angle. The reason is strength; the lower the helix angle, the stronger the gear. But Nash agrees that for street use a 20-degree angle is probably overkill and there are plans to change 4th gear to a quieter 25-degree angle.

Our other complaint is shift effort. The standard Corvette linkage is noted for being quick and positive with effort that is uniquely light for the size of the gears being shifted. Not so the 5-speed. It felt more like a racing transmission: Lever travel fore and aft and side to side was reduced at the expense of considerably higher effort and a very notchy feel. Chalk this up to Nash's drag racing background and be advised that linkage more suited to street driving is being developed.

Why didn't Nash design 5th gear as an overdrive instead of 1.00:1? The answer is strength and durability. The combination of a low numerical axle ratio and high numerical transmission gearing will live longer than the reverse combination because as the final drive ratio increases numerically, the size of the pinion gear decreases and the number of pinion teeth increases. Also, although engine speed is reduced with an overdrive transmission, everything back of the engine is still turning high revs. And because most of today's cars are being built with low numerical axle ratios for reasons of improved fuel economy and lower emissions, it means no change in final drive ratio is necessary to take advantage of the Nash 5-speed. The 4 + 1 lists for $995 (the racing version costs $1100) and is available from Doug Nash Equipment & Engineering Corp., 111 Century Court, Franklin, Tenn. 37064, 615 790-2500.